1. Field of the Invention
The present invention relates to an electrophotographic image forming device, such as a laser beam printer, a facsimile machine, and a copy machine. The present invention particularly relates to an image forming device that develops an electrostatic latent image on a photosensitive drum using toner transported by a developing roller to a nip portion between the photosensitive drum and the developing roller.
2. Description of the Related Art
Impression development is widely used as a developing method in electrophotographic image forming devices. Impression development uses a roller, which is formed from an electrically conductive and resilient material, as a developing roller for carrying toner. The developing roller is pressed against the photosensitive drum to perform development.
A corona charge device is provided for charging the entire surface of the photosensitive drum. An exposure means is provided for selectively exposing the charged surface of the photosensitive drum to form an electrostatic latent image on the surface of the photosensitive drum. A predetermined developing electric field is developed between the developing roller and the photosensitive drum. Therefore, when toner born on the surface of the developing roller contacts the surface of the photosensitive drum, toner born on the developing roller moves toward and develops the electrostatic latent image.
Either positively charged toner or negatively charged toner can be used in impression development. Conventionally, negatively charged toner is mainly used because of stability in its charge characteristic.
Printers that expose images using a laser beam, for example, use what is called reverse development to form toner images, wherein the polarity of charge on the photosensitive drum matches the polarity of charge of the toner. Therefore, when the toner used in development has a negative polarity, the photosensitive drum must also be charged with a negative polarity. Accordingly, the polarity of voltage discharged by the corona charge unit must have a negative polarity.
However, when a corona charge unit is used to charge the surface of the photosensitive drum, the corona charge unit ionizes atmospheric oxygen molecules (O.sub.2) so that ozone (O.sub.3) is generated. This is particularly the case, when the corona charge unit discharges a negative polarity charge, wherein the amount of ozone generated can be ten times greater or so than when the corona charge unit discharges a positive polarity charge. This is undesirable from an environmental view point.
To overcome this problem, it is conceivable to use a positively charging toner in a printer. Such a printer can use a corona charge unit that discharges voltage having a positive polarity so that the amount of ozone generated can be reduced
However, it is impossible to completely remove the electric potential from the surface of the photosensitive drum of this printer. Therefore, when reverse development processes are applied, thin lines and independent dots can be difficult to reproduce.
With the exception of expensive and highly precise laser optical systems, variation and imprecision in the laser optical system will increase spot diameter of laser light for irradiating the photosensitive body to much greater than the theoretical value. Therefore, the power of the laser light will be dispersed over a larger area. As a result, the laser light will be incapable of sufficiently reducing the electric potential of the photosensitive drum to produce an appropriate electrostatic latent image.
Thin lines and independent dots are especially difficult to reproduce when using impression development with non-magnetic single component toner, which normally is uninfluenced by the edge effect.